Three basic techniques exist for removing heat from various substances to effect refrigeration and/or freezing: natural refrigeration, mechanical refrigeration and cryogenic freezing. In natural refrigeration, use is made of ice whereby the heat to melt the ice is extracted from the substance to be refrigerated. The limitation of this technique is the freezing point of water: hence natural freezing is of little practical value in modern mass-production freezing plants.
Mechanical refrigeration generally operates on the principle of a liquid-to-gas, gas-back-to-liquid cycle. Numerous methods and systems for providing mechanical refrigeration have been developed: vapor-compression, absorption, steam-jet or steam-ejector, and air. Each cycle operates between two pressure levels, and all except the air cycle use a two-phase working medium which alternates cyclically between the liquid and vapor phases.
The most common form of mechanical refrigeration incorporates the vapor-compression cycle. The vapor compression cycle consists of an evaporator in which a liquid refrigerant (such as ammonia or Freon) boils at low temperature to produce cooling, a compressor to raise the pressure and temperature of the gaseous refrigerant, a condenser in which the refrigerant discharges its heat to the environment, and an expansion valve through which the liquid expands from the high-pressure level in the condenser to the low-pressure level in the evaporator.
Vapor compression mechanical refrigeration systems are commonly used for the mass production of frozen foods. Foods having high water content, such as fish, tomatoes and citrus fruits, however, cannot be satisfactorily frozen with mechanical refrigeration systems since the relatively low freezing rates give rise to ice crystal growth, which ruptures the cell walls and tissues and causes food to assume a mushy consistency upon thawing. Slow freezing also causes a loss of moisture and of volatile oils, which impairs the flavor of the product and causes undesirable shrinking thereof.
In mass production freezing plants utilizing simple mechanical refrigeration systems such as the vapor-compression system described above, problems arise when the heat load inside the refrigerated space exceeds the refrigeration capacity of the mechanical refrigeration system. Mechanical refrigeration systems are inherently limited by the size and efficiency of the four basic components, reflected by the ability of the evaporator to absorb heat quickly. In mass-production freezing plants, the inability of a mechanical refrigeration system to absorb heat quickly causes production "bottlenecks," which result in decreased production rates and/or inadequately frozen products.
The third technique for removing heat from various substances to effect freezing involves the use of a cryogen. In cryogenic freezing, a cryogen, such as liquid carbon dioxide, liquid nitrogen, liquid air or any other substance having a normal boiling point below -100.degree. F., is used to effect freezing by either direct or indirect contact with the material to be frozen. With cryogenic liquids, under carefully controlled conditions, freezing rates can be obtained which are so fast that high-water content products can be frozen in substantially amorphous form whereby little or no collapse of the internal structure will occur upon thawing. One important advantage of cryogenic freezing is that it reduces food shrinkage since the quick freezing of the water content limits the loss of moisture.
While cryogenic freezing may be effective with certain foods, for instance, where the food has a high water content and is of somewhat delicate internal structure, the change in internal temperature and the resultant formation of ice which expands the structure may give rise to thermal shock damage, as a consequence of which the product is cracked and otherwise mutilated.
Most mass production freezing plants in existence today incorporate mechanical refrigeration systems having limited refrigeration capacities. The existing mechanical refrigeration system is generally not capable of maintaining an optimal operating temperature if the heat load inside the refrigerated space exceeds the refrigeration capacity of the mechanical refrigeration system. In an attempt to accelerate the freezing action of an existing mechanical refrigeration system, i.e., increase its refrigeration capacity or the refrigeration capacity of the entire "cooling operation," additional mechanical refrigeration units could be added. This, however, would require the investment of substantial capital and may not prove cost-efficient. The lack of adequate factory space and problems with retrofitting existing mechanical systems may also prevent the addition of supplemental mechanical refrigeration units. Further, since most production facilities already have a substantial investment in their existing mechanical refrigeration units, replacing them with pure cryogenic systems would be timely and cost inefficient.
Arrangements have been proposed, such as that disclosed in U.S. Pat. No. 3,531,946, to combine a mechanical refrigeration system with a cryogenic system. In this "hybrid" cooling system, products are conveyed on a belt into a thermally-insulated chamber having fans which circulate cold air from mechanical refrigeration coils located below the conveyor belt. The products are sprayed with a cryogen after entering the chamber, the cryogen serving to superficially freeze the food to produce a thin ice glaze thereon, serving to prevent moisture loss from the food.
This cooling system, however, cannot function as designed due to inherently low internal operating temperatures caused by the operation of the cryogen system. The introduction of a cryogen having a temperature below -100.degree. F. into a mechanically refrigerated space causes the temperature in the refrigerated space to drop significantly below the operating temperature of the mechanical refrigeration system, which, depending on the liquid refrigerant, is generally no lower than -40.degree. F. Since there is no heat to be absorbed by the mechanical evaporator, the mechanical refrigeration system cannot function as designed and the mechanical refrigeration system shuts down.
Additional cooling systems which combine mechanical and cryogenic systems are also known. A two-stage freezing apparatus is disclosed in U.S. Pat. No. 4,517,814 in which a product is transported by a conveyor belt system through a liquid cryogen bed and thereafter to a conventional mechanical freezer. U.S. Pat. No. 4,972,681 discloses a freezing device in which products are first treated with a liquid cryogen, then moved via a conveyor system to a second area where fans are used to circulate the gaseous cryogen, and then moved to a third area which houses a conventional air freezer.
Although the concept of combining a mechanical refrigeration system and a cryogen system has been known in the art for some time, there still remains a need for a cooling operation or system which can increase the refrigeration capacity of a mechanical refrigeration system while not affecting the proper performance of the mechanical refrigeration system, thereby increasing the operating productivity rate in the presence of high heat load conditions, but which is relatively simple to install, which avoids the problems associated with retrofitting existing mechanical refrigeration systems, which does not require additional refrigerated or production space and which can operate simultaneously with a continuously operating mechanical refrigeration system.